Stay on target

In the history of complex life on earth — more than half a billion years — there have been five mass extinctions. Each of them is clearly evident in the fossil record, and leaves dramatic chemical markers in the soil and rock layers below. All of them also represent significant jolts to the fundamental mechanisms that keep life on this plant going.

Plants, for example, are constantly hard at work cranking out the oxygen that the rest of us need to breathe. If something happens to that supply — like most plants on earth suddenly dying off — then we’d be screwed.

Up to now, there’s been strong evidence that, because of human activity, we are sprinting towards the sixth great mass extinction. New data suggest that we’re closer to the brink than we might think, and humanity is dangerously close to crossing critical thresholds.

Mass Extinctions, be they caused by a meteor, or an ancient supervolcano, or anything else, are invariably preceded by massive jolts to the carbon cycle — a network of natural systems that moves carbon molecules around the planet. In a new paper in Science Advances, professor of geophysics at MIT, Daniel Rothman, asserts that there are two types of carbon disruptions that cause mass die-offs.

The first says that if the carbon cycle changes over long periods — thousands or millions of years– extinctions occur when ecosystems cannot adapt. For faster shifts, the magnitude, not the rate of change, determines how likely an extinction event will be.

Using models, Rothman asserts that humans will have crossed that marker by 2100. Carbon, he says, will build up in the ocean. After we dump about 310 gigatons of extra carbon into the seas, we will have hit a point that would not only be disastrous climatologically, but systemically. Over the next few thousand years, we could expect the vast majority of species on earth to die off. While life would surely adapt, such an event would amount to permanent, irreparable damage to Earth’s biodiversity. And, it means that human industry would be about as destructive as the largest of space rocks.

“This is not saying that disaster occurs the next day,” Rothman told Phys.org. “It’s saying that, if left unchecked, the carbon cycle would move into a realm which would be no longer stable, and would behave in a way that would be difficult to predict. In the geologic past, this type of behavior is associated with mass extinction.”
We could be facing trouble much earlier, though, and Rothman worries that the world may already be in dire straits by 2100.

Rothman’s paper focused exclusively on the carbon cycle. It’s a system that balances the chemical output of photosynthesis — plants’ primary means of making energy — and respiration — the mechanism that animals and other complex life uses. Over time, carbon falls to the bottom of the ocean — either due to currents or the steady sifting of dead material and detritus from the waters above. That locks it away for millions of years, and works a planet-wide storage system.

Over the past few hundred years, humans have been dredging up a lot of that excess carbon in the form of fossil fuels, and pumping it back out into the atmosphere. That’s more than overwhelmed Earth’s ability to sequester the element.

Rothman used data from the Intergovernmental Panel on Climate Change that predicted how much carbon will hit the oceans in the coming decades. The IPCC looked at four possible outcomes for human carbon emissions, estimating the impact of each. The worst case scenario would dump over 500 gigatons of carbon into the oceans by 2100 — well over the critical threshold. The best case scenario wasn’t that much better though, adding a whopping 300 gigatons. If these data prove accurate, we’re in big trouble. Not that we already weren’t, but… yeah.